Get the libpython library name from Python

CMake's find_library can return a static library in some conditions
(certainly if the shared object isn't present; I'm not sure if there
are other conditions as well), which we don't support (and fails).

This improves the libpython detection by getting the shared library name
directly from Python rather than letting CMake guess (by prepending
`lib` and appending library suffixes).

This should, at least, prevent the embedded build from failing as in
issue pybind#1159 -- instead it should either find the proper shared library,
or else result in a compilation failure from the missing library.

Author: jagerman

tools/FindPythonLibsNew.cmake

@@ -85,6 +85,7 @@ print(struct.calcsize('@P'));
 print(s.get_config_var('LDVERSION') or s.get_config_var('VERSION'));
 print(s.get_config_var('LIBDIR') or '');
 print(s.get_config_var('MULTIARCH') or '');
+print(s.get_config_var('LDLIBRARY'));
 "
     RESULT_VARIABLE _PYTHON_SUCCESS
     OUTPUT_VARIABLE _PYTHON_VALUES
@@ -112,6 +113,7 @@ list(GET _PYTHON_VALUES 6 PYTHON_SIZEOF_VOID_P)
 list(GET _PYTHON_VALUES 7 PYTHON_LIBRARY_SUFFIX)
 list(GET _PYTHON_VALUES 8 PYTHON_LIBDIR)
 list(GET _PYTHON_VALUES 9 PYTHON_MULTIARCH)
+list(GET _PYTHON_VALUES 10 _PYTHON_LIBRARY_NAME)
 
 # Make sure the Python has the same pointer-size as the chosen compiler
 # Skip if CMAKE_SIZEOF_VOID_P is not defined
@@ -165,7 +167,7 @@ else()
     # Probably this needs to be more involved. It would be nice if the config
     # information the python interpreter itself gave us were more complete.
     find_library(PYTHON_LIBRARY
-        NAMES "python${PYTHON_LIBRARY_SUFFIX}"
+        NAMES ${_PYTHON_LIBRARY_NAME}
         PATHS ${_PYTHON_LIBS_SEARCH}
         NO_DEFAULT_PATH)